1. Field of the Invention
The present invention relates to an image exposure apparatus with a different angle incident light optical system which comprises a plurality of light sources for emitting light beams of different narrow band wavelengths at different angles respectively, a deflection optical system for receiving on its same reflecting surface and reflecting the light beams in the main scanning direction to scan a photosensitive material at predetermined intervals along the scanning lines for exposure, and a single optical sensor unit located outside of the light path for exposure extending from the deflection optical system to the photosensitive material for providing the timing of start point synchronization for exposure actions along the main scanning direction.
2. Description of the Related Art
Digital image exposure apparatuses have been proposed for actual use where a group of light beams of red, green, and blue narrow band wavelengths modulated by pixel data of, e.g., each digital color image which has been received from a film scanner scanning a photographic film or shot by a digital camera and saved in a memory are directed to and fallen on a photosensitive material such as a color printed sheet of which the spectral sensitivity depends on the wavelengths of exposure light for recording and reproducing the color image.
The digital image exposure apparatuses employ commonly a scanning exposure technique for exposing the photosensitive material, which is conveyed in a sub scanning direction oriented at a right angle to the main scanning direction, to the light beams deflected to scan along the main scanning direction.
It is essential for the exposure scanning on the color photosensitive material to scan the light beams of different narrow band wavelengths in blue (B), green (G), and red (R) colors along the main scanning line on the photosensitive material as precisely as possible for printing the three primary colors; yellow (Y), magenta (M), and cyan (C). In practice, as shown in FIG. 11, three light beams of R, G, and B colors emitted from their respective light sources 7R, 7G, and 7B are combined by the action of light combining prisms 54R, 54G, and 54B and then received by a deflection optical system 8 for exposure of the photosensitive material. Although being used widely, this method however requires a set of fairly expensive optical components for combining the light beams and increases the number of the entire components to be assembled as yet having a limitation on the geometrical arrangement of the light sources and will hence be unfavorable for reduction of the overall size.
A type of the conventional image exposure apparatuses which is minimized in the production cost and the overall size is illustrated in FIG. 1 where light beams of blue (B), green (G), and red (R) colors emitted from the light sources 7R, 7G, and 7B such as light emitting diodes or semiconductor lasers are focused at different locations of the main scanning line on a photosensitive material with the use of a different angle incident optical system for scanning at predetermined intervals for exposure of the photosensitive material. This type requires the light beams to be timed for starting the exposure action on the photosensitive material or the timing of start point synchronization to be measured by optical sensor units respectively which are located outside of the light path for exposure, thus controlling the exposure timing of each light beam.
If the optical sensor units are provided corresponding to the three light beams, the number of the components will increase and thus interrupt the reduction of the overall size. For compensation, the use of a single optical sensor unit has been introduced. For producing the yellow (Y), the magenta (M), and the cyan (C), the corresponding light beams of blue (B), green (G), and red (R) are scanned at different intensities over the photosensitive material as shown in FIG. 9. This may however permit the single optical sensor unit, which has once been adjusted to a desired degree of the sensitivity, to successfully measure the red (R) of the light beam which is the highest in the intensity but fail to detect the green (G) and blue (B) which is lower in the intensity than the red (R). If any of the three light beams is not detected, the reproduction of an image will be unsuccessful.
As disclosed in Japanese Unexamined Patent Publication No. Hei-5-199372, a modification is proposed where a group of light sources are adapted for allowing the optical sensor unit to receive particular one of the light beams to be irradiated at the highest intensity precedent to the other color light beams. When all light beams are emitted at once from the light sources, their start point synchronization for exposure actions is initiated upon the optical sensor unit detecting the highest intensity of the light beam. More specifically, the start point synchronization for exposure actions of the beams of light over a photosensitive material is triggered by a detection signal of the optical sensor unit produced upon detecting the light beam of the highest intensity at the top.
The modification of the conventional technique disclosed in this invention is based on the notable fact that the duration from the optical sensor unit detecting the beam of light at the highest level to the arrival of each beam of light at the exposure start point on the photosensitive material is calculated from the distance between the location of the optical sensor unit and the exposure start point, the interval between the beams of light along the main scanning line determined by the location of the light sources, and the main scanning speed determined by the length of the light path and the rotating speed of the polygonal mirror in the deflection optical system. The timing for starting the exposure action with each beam of light is thus determined using a delay circuit which is provided for timing the arrival of the corresponding beam of light by delay after the optical sensor unit detects its start point detecting signal. However, this produces the following problems.
The light sources have to be positioned so that the beam of light at the highest intensity is first received by the optical sensor unit precedent to the other color beams of light. Accordingly, as the freedom for positioning the light sources is declined, the arrangement of design will hardly be flexible and favorable in the reduction of the overall size. When the light sources for emitting beams of light of narrow band wavelengths are embodied by laser devices, as shown in FIG. 6, the red color (R) of light at a higher output level is emitted from a semiconductor laser and the green (G) and blue (B) of light at lower output levels are emitted from SHG (second harmonic generation) laser devices in general. Those laser devices however are different in the package size and their positioning may largely affect the overall dimensions of the apparatus. Also, the size of a printer in which the conventional image exposure apparatus is installed will be reduced with much difficulty.
In practice, the delay time may be different between the setting and the actual length due to variations in the location of the light sources and the temperature characteristics of the modulating elements. If worse, the reproduced image will have color displacement. It is hence necessary to add the assembly process with an extra step for correcting any delay caused by machine differences.
Moreover, if the optical axis of any acousto-optic device employed for modulating the beam of light with a pixel data is slightly dislocated by the effect of temperature change or mechanical impact, it will critically affect the exposure action. It is thus needed to constantly examine the beams of light for its correctness. On the contrary, this will unfavorably require an extra sensor unit for the examination in the conventional image exposure apparatus.